Detection device for hazardous materials

ABSTRACT

A detection device that is activated by the interaction of a hazardous  chcal with a coating interactive with said chemical on an optical fiber thereby reducing the amount of light passing through the fiber to a light detector. A combination of optical filters separates the light into a signal beam and a reference beam which after detection, appropriate amplification, and comparison with preset internal signals, activates an alarm means if a predetermined level of contaminant is observed.

CONTRACTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. DE-AC07-76ID01570 between the U.S. Department of Energy and EG&GIdaho, Inc.

BACKGROUND OF THE INVENTION

This invention relates to a device that can be worn or carried by anindividual, and is capable of detecting air-borne hazardous chemicalssuch as hydrazine, at extremely low concentrations.

Hydrazine is a colorless, fuming hygroscopic liquid that is misciblewith alcohol and water. A strong reducing agent, it is highly toxic byingestion, inhalation and skin absorption and is a strong irritant toskin and eyes. In addition to being a carcinogen, it poses a severeexplosion hazard when exposed to heat or flame or by reaction withoxidizing materials. For these reasons its recommended tolerance in airis on the order of 100 parts per billion (ppb).

Hydrazine has a multitude of uses, most notably in rocket fuels,agricultural chemicals, drugs, metal and glass plating, fuel cells,solder flux, explosives, photographic developers and as a corrosioninhibitor in water-cooled nuclear power plants. Because of this widerange of product uses and its deadly nature, it is of paramountimportance to carefully monitor for the presence of hydrazine in thoseplaces where hydrazine may be present in the air due to leakage,spillage, or other means.

SUMMARY OF THE INVENTION

The present invention is a detection device utilizing fiber optics asthe detecting medium. By coating a fiber optical fiber with an aldehydesusceptible to a color change when exposed to hydrazine, lighttransmission and/or absorbance through the fiber is altered. Light froma broadband source, such as a tungsten-halogen lamp, is passed through afocusing lens onto the first end of the optical fiber. Light emittedfrom the second end is projected onto a combination of filters whichpermits only light of a desired frequency to pass. The device isinterconnected to a sensing diode such that any variation in lighttransmission, above or below a pre-determined threshold level, willactivate an audible and/or visible alarm. The apparatus may beconfigured into a package approximately the size of a pocket calculatorand can be operated from a small battery pack.

Detection of other hazardous materials may be effected by applyingdifferent coatings to the fiber susceptible to a color change whenexposed to the material to be detected. The filter system and sensingdiode are selected depending upon the particular light frequencynecessary for that detection.

The reaction set forth below is applicable for the three types ofhydrazine commonly used as rocket fuels: hydrazine (H₂ NNH₂), methylhydrazine (CH₂ NHNH₂) and unsymmetrical dimethyl hydrazine ((CH₃)₂NNH₂). Each of these will react with an aldehyde coating on an opticalfiber and produce a color change in the coating. The degree of colorchange and rate of reaction will usually vary with different aldehydes.After testing a number of different aldehydes, the optical coatingselected for use on the sensor of the present invention was4-nitrobenzaldehyde (NO₂ C₆ H₄ CHO). This particular aldehyde gave thefastest response and most distinctive color change (colorless to brown)when in the presence of hydrazine. Using methylhydrazine as theperceived hazardous chemical, the applicable reaction is:

    NO.sub.2 C.sub.6 H.sub.4 CHO+CH.sub.3 NHNH.sub.2 →NO.sub.2 C.sub.6 H.sub.4 CH═NNHCH.sub.3 +H.sub.2 O                     (1)

To secure the aldehyde to the optical fiber, a solution of 1%polyethylene oxide by weight in 99% methylene chloride (CH₂ Cl₂) byweight was prepared. To this solution was added 4-nitrobenzaldehyde toprepare a 1X10⁻⁴ molar solution. Drawing the optical fiber through thissolution deposited a film having an approximate thickness of 1 micrononto the optical fiber.

The refractive index of the polymer was determined from the literatureto about 1.454 (slightly lower than that of the quartz fiber index of1.46), which permitted the evanescent lightwave to interrogate thealdehyde molecules on the surface as they reacted with the hydrazinevapors in the air. If the refractive index of the coating is less thanthat of the fiber, internal reflectance occurs and most of the lightremains within the fiber core. That portion of the internal lighttransmitted by the fiber which penetrates into and is absorbed by thefiber coating is referred to as the evanescent wave.

As the reaction between the aldehyde in the coating and the airbornehydrazine occurs, a portion of the light is absorbed by the coating,reducing the amount of light transmitted by the fiber. The change inlight intensity can be measured by the detecting diode. When thedetecting diode measures a decrease in light intensity greater than thatpredicted based upon a control situation, the detecting diode activatesa visual and/or an audible alarm to alert an operator that hydrazine ispresent in the air in an undesirable concentration.

The experimental apparatus utilized herein comprised a 58 liter testchamber containing a heat plate and fan for instantaneous vaporizationand circulation of the hydrazine vapors. Placing the detector containingthe coated optical fiber in the test chamber allowed detection ofhydrazine down to about 12 ppm in an approximate 5-minute time period.Further refinements are expected to lower the detection level to the lowparts-per-billion range, at or below the 100 ppb tolerance level. Thesensitivity of the coating can be increased by increasing theconcentration of aldehyde on the fiber by increasing the surface area ofthe optical fiber, or by decreasing the coating thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the major components of the presentinvention;

FIG. 2 illustrates the hydrazine detector portion of the presentinvention;

FIG. 3 is a schematic diagram illustrating the electrical, mechanicaland optical apparatus of the present invention;

FIG. 4 is a graph of absorption of 4-nitrobenzaldehyde versusillumination wavelength before and after exposure to hydrazine;

FIG. 5 is a graphic illustration of detector output signal intensityversus time at 60 ppm hydrazine; and

FIG. 6 is a graphic illustration of detector output signal strengthversus time at 25 ppm hydrazine.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates diagrammatically the major components of thehydrazine detection device 10 of the present invention. Power supply 12provides +15, -15 and +4.8 volts to electronic circuit board 14 which inturn powers the sensor assembly 16 and detector assembly 18 by means ofcables 20. The power supply input can be optionally energized by 110volts AC power 21. Sensor assembly 16 is comprised of lamp housing 22,lens focus means 24, a first X-Y micrometer stage 26 and optical fiber28 which is chemically treated with an aldehyde coating adapted to reactwith hydrazine. The reaction at the surface of the optical fiber causeslight transmitted through the fiber to be absorbed into the coating,such that less light is available to enter and be detected by thedetector assembly 18.

The detector assembly 18 is illustrated in greater detail in FIG. 2.Optic fiber 28 comprises a conventional quartz fiber having a coating 31of an aldehyde, such as 4-nitrobenzaldehyde. Fiber 28 is attached to asecond X-Y micrometer stage 40 which fine tunes the alignment of thelight beam emitted from fiber 2 into the mirror housing 42 and ontofocusing lens 44. The micrometer stages 26, 40 are conventionalapparatus' well-known to those skilled in this art. For example, amicrometer stage marketed by The Newport Co. under the name FiberopticPositioner has been found useful in this embodiment. Light beam 46passes through the focusing lens 44 and thereafter impinges on a beamsplitter in the form of a conventional dichroic mirror 48, whereupon afirst light beam signal portion 50 is separated from a second light beamsignal portion and directed through a filter 52 onto signal photodiode54. The electrical output from diode 54 is then transmitted throughconductors 56 to signal amplifier 58. The output from amplifier 58 istransmitted by conductor 60 to the electronic circuit board 14 (FIG. 1)for further processing.

The second light beam signal portion (or reference portion) 62 of beam44 is directed through filter 64 to reference photodiode 66. Theelectrical output of diode 66 is transmitted through conductors 68 toreference amplifier 70. The amplified reference signal is transmittedthrough conductor 72 to electronic circuit board 14 (FIG. 1) similar tothe amplified signal portion 60 described above.

It is to be understood that the aldehyde coating applied to the opticalfiber can take many forms based upon the particular circumstances, andthat the coating thickness may vary. While it is contemplated that thethicness will be on the order of about 1 micron, it is expected thatgreater sensitivity of the device can be obtained with a thinnercoating. It should also be appreciated that the filters 52, 64 areconventional state-of-the-art filters selected such that particularwavelengths of light are emitted therethrough, which are thereafterassimilated at circuit board 14. For example, Applicants have found thata 360 nanometer (nm) band pass filter 52 and a 700 nm band pass filter64 produces exemplary results in the apparatus of the present invention.

Operation of an experimental hydrazine detection device 10 of thepresent invention is illustrated by reference to the schematic diagramofFIG. 3. Power supply 12 distributes +15, -15 and 4.8 volts DC to thecircuit board 14, which in turn provides 4.8 volts to sensor assembly 16through cables 20. The assembly 16 comprises light source 80, such as atungsten-halogen lamp, which directs light 84 onto first lens 82. Thelight beam 83 exiting the lens 82 is focused by lens 82 through thefirst X-Y micrometer stage 26 into the coated optical fiber 28. Thesensor assembly 16 is placed in an airtight container or test chamber 85for testing. A predetermined amount of hydrazine (for example, an amountsufficient to result in a final concentration of 25 ppm or 60 ppm withincontainer 85) is introduced into the test chamber 85. The hydrazinewithin the test chamber comes into contact with and reacts with thealdehyde coating around the optical fiber 28. A representative reactionof the airborne hydrazine with the aldehyde coating is illustrated inequation (1) above, and results in absorption of light in the aldehydecoating from fiber 28, color from essentially colorless (thereforelittle or no absorbance of light) to brown. The brown coating reducesthe of light transmitted through the optical fiber 28 to the detector18.

The light from fiber 28 passes through the second X-Y micrometer stage40 and is focused by lens 44, with beam 46 being split at dichroicmirror 48. The reference portion 62 of beam 44 is a constant againstwhich the signal portion 50 of beam 44 is measured. Reference beam 62 isdirected through a 700 nm band pass filter 64 and thence onto referencephotodiode 66. The output signal is conducted to reference amplifier 70.The signal portion 50 of beam 44 is directed through a 360 nm filter 52,the wavelength chosen to maximize the amount of light absorbed by thealdehyde coating.

As the "browning" reaction between the aldehyde coating and hydrazineoccurs, absorbance of light by the coating increases and less light istransmitted through cable 28 to the mirror 48 and signal photodiode 54.The reduced signal voltage is transmitted through conductor 60 to alarmmeans 88, including detector comparator 86, which is programmed to sensea change in the difference between the variable signal voltage enteringvia conductor 60 and an internally adjustable reference signal.

The detector comparator 86 is interconnected with a low-light comparator90 via conductor 92 and an appropriate resistor 94. The signalcomparison between the comparators 86, 90 is utilized to eliminate aspurious alarm due to low light level entering the device at 46, asopposed to low detector level signal at 54 resulting from the presenceof hydrazine.

A third low-voltage comparator 96 receives input from the 4.8 volt DCpower supply via cable 20. Comparator 96 senses a difference between the4.8 volt signal voltage and an internally adjustable preset voltage,which in this case would normally be 4.8 volts. A perceived voltage at96 (i.e. less than 4.8 volts) reduces the intensity of light 80, therebyreducing the intensity of light transmitted through detector assembly 18to photodiode 54. Therefore, while the comparators 86 and 90 mayindicate low light received (indicating the presence of hydrazine) suchsignals may be ignored if the comparator 96 indicates inadequatevoltage, and therefore prevents accepting the results perceived atcomparators 86, 90.

The device disclosed above can be modified to provide that the intensityof light beam 44 is immaterial, such that the comparison is made betweenthe actual voltage and the actual intensity of beam 44.

In each case when the variable signal level monitored by one ofcomparators 86, 90, 96 differs from the internally preset signal level,the resultant output activates alarm means, such as detector lightemitting diode (LED) alarm 100, low-liqht LED alarm 102 or low-voltagealarm LED 104. An audible alarm may also be incorporated with any of thecomparators.

The low-light comparator 90 can be internally adjusted to detect anincrease in light as well as the aforementioned reduction in light. Inthe event of, for example, a broken seal in the mirror housing 40allowing unwanted light into the housing, such light would adverselyaffect calibration and should be detected. Additionally, changing thealignment of lens 46 can cause an increase in light affectinqcalibration of the device.

In addition to the visual LED alarms, the signal from each comparatorcan be connected to a quad bilateral switch/timer 106 that activatespiezoelectric buzzer 108 to warn an operator of other than optimalconditions. Such conditions (other than the expected detection ofhydrazine at the predetermined level) inoperative sensor preventingoperation as designed (such as fade-out of the light signal) or breachof the sensor container due to dropping, or inadequate voltage (deadbatteries).

FIG. 4 illustrates the effect of exposure of 4-nitrobenzaldehyde (thepreferred fiber coating) to hydrazine. A coating of 4-nitrobenzaldehydeentrapped in polyethylene oxide polymer was exposed to hydrazine underambient conditions. The relative values of light absorption of4-nitrobenzaldehyde before (curve 112) and after (curve 110) exposure tohydrazine, were obtained to determine that wavelength of light wheremaximum absorbance occurred. The difference between the two curvesmaximizes at 114, representing the absorbance occurring at a wavelengthof about 360 nm. The band pass value of the detecting filter 52 wastherefore set at this wavelength. Selection of this wavelength can beseen to maximize detector sensitivity between signals as shown at 116.

Signal strength measured by photodiode 54 versus time of exposure ofcable 28 within test chamber 85 to a 60 ppm concentration of hydrazineis illustrated in FIG. 5. A 10% drop in voltage (from 2.2 to 2.0 volts)occurs at the knee 122 after an exposure time of 5 minutes at thisconcentration.

The effect of a reduced concentration (25 ppm) of hydrazine in theexperimental apparatus of FIG. 3 is illustrated in FIG. 6, wherein theknee of the curve occurs at point 128, representing an approximate 10%reduction in signal strength. Such reduction occurs after an exposuretime about 14 minutes. Applicants have determined that concentrations aslow as 12 ppm are detectable in an approximate 5-minute exposure timeusing the apparatus of FIG. 3. It is anticipated that exposure timessignificantly less than 5 minutes can be achieved with processoptimization. It is therefore anticipated that further refinements canreduce the detection level to the parts-per-billion range for reasonableexposure times. For example, by increasing the concentration of aldehydein the coating, or by increasing the diameter of the optical fiber(thereby increasing the surface area), or by decreasing the thickness ofthe coating, the sensitivity of the process described herein can beincreased.

While the invention has been described herein with particular referenceto a coating of 4-nitrobenzaldehyde as the preferred aldehyde fordetecting hydrazine, it is to be appreciated that other aldehydecoatings are operative in the practice of this invention. For instance,p-bromobenzaldehyde and salicylaldehyde are acceptable coatingconstituents.

While the apparatus and process of the present invention have beendescribed and illustrated herein with reference to hydrazine detection,it is to be understood that the process is equally susceptible for thedetection of other hazardous chemicals. So long as a color-sensitivecoating reactive with the chemical to be detected can be applied to anoptic fiber, such process can be utilized. Therefore, a coating ofphenoxazine can be used to detect ozone or nitrogen dioxide. In thepresence of ozone, the coating turns a dull brown color and withnitrogen dioxide a red-orange color. Similarly, a coating oftetracyanoethylene in the presence of light aromatic hydrocarbonschanges color--a bright yellow in the presence of benzene, orange in thepresence of toluene and red in the presence of m-xylene. In its broadestembodiment, the present invention is the application of a coating to anoptical fiber, the coating being chosen to interact with an airbornehazardous chemical. The interaction involves a color change of thecoating causing a decrease in the amount of light transmitted throughthe fiber, which can be quantified with reference to a reference lightsource.

While a preferred embodiment of the invention has been disclosed,various modes of carrying out the principles disclosed herein arecontemplated as being within the scope of the following claims.Therefore, it is understood that the scope of the invention is not to belimited except as otherwise set forth in the claims.

We claim:
 1. A hydrazine detection device comprising:a. a power supplyinterconnected to a light source; b. sensor means interconnected withthe light source and having an optical fiber coated with a materialcomprising polyethylene oxide and 4-nitrobenzaldehyde capable ofreacting to the presence of hydrazine; c. light detecting means; and d.alarm means to indicate a change in light transmitted by the opticalfiber responsive to the presence of a predetermined level of hydrazine.2. The device as recited in claim 1, wherein the refractive index of thematerial coating the fiber optic cable is substantially similar to therefractive index of the cable.
 3. The device as recited in claim 1,wherein the optical fiber is coated to a depth of less than one micron.4. The device as recited in claim 1, wherein the material comprises 1%polyethylene oxide by weight and 99% methylene chloride by weight towhich is added 4-nitrobenzaldehyde to make a 1×10⁻⁴ molar solution. 5.The device as recited in claim 1, wherein the sensor means furthercomprises:a. a tungsten-halogen light; b. a focusing lens; and c. afirst X-Y micrometer stage wherein the tungsten-halogen light is focusedby the focusing lens into a first end of said optical fiber.
 6. Thedevice as recited in claim 5, wherein the light detecting means furthercomprises:a. a housing having:i. a dichroic mirror; ii. a focusing lens;iii. a second X-Y micrometer stage; and iv. a first and second lightfilter; b. a signal photodiode interconnected with a signal amplifier;and c. a reference photodiode interconnected with a reference amplifier.7. The device as recited in claim 6, wherein the first light filter is abeam-pass filter of about 360 nanometer wavelength and the second lightfilter is a beam-pass filter of about 700 nanometer wavelength.